In the New Testament, Paul the apostle says under the Holy Spirit's leading to bring the tithes on the first day of the week into the church for the support of the ministry, the needy, etc. 1 Corinthians 16:1, 2
16:1 Now concerning the collection for the saints, as I have given order to the churches of Galatia, even so do ye. 16:2 Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come.
Using food to pay tithe if there is no money or if it is equivalent to the amount of money you have determined is the percentage you wish to tithe is not forbidden. God wants a sincere heart in giving to back to Him a portion of what He has generously blessed us with and prospered us. He also wants a cheerful heart in giving to Him and these are the main points.
When Jesus said to pray in secret and go in to one's prayer closet, He did mention to shut the door so it does mean to go to real or literal place, room, etc. where you can shut yourself away from the world's eyes and pray privately to the Lord after which He promised to bless us openly when we did just that.
6:5 And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward. 6:6 But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.
It can also have an added application in praying secretly within one's mind and heart, but the original and primary context and application is in relation to a secret place where one can pray silently or loudly, opening our mouth and lips to God.